1. Field of the Invention
The present invention relates to a plasma processing apparatus, and in particular to a plasma processing apparatus having therein an electrode that is connected to a DC power source.
2. Description of the Related Art
Parallel plate type plasma processing apparatuses are known that have a substrate processing chamber that has therein a processing space and into which is transferred a wafer as a substrate, a lower electrode that is disposed in the substrate processing chamber and is connected to a radio frequency power source, and an upper electrode that is disposed such as face the lower electrode. In such a plasma processing apparatus, a processing gas is introduced into the processing space, and radio frequency electrical power is applied into the processing space between the upper electrode and the lower electrode. When a wafer has been transferred into the processing space and mounted on the lower electrode, the introduced processing gas is turned into plasma through the radio frequency electrical power so as to produce ions and so on, and the wafer is subjected to plasma processing, for example etching processing, by the ions and so on.
In recent years, with an aim of improving the plasma processing performance, plasma processing apparatuses in which the upper electrode is connected to a DC power source so that a DC voltage is applied into the processing space have been developed. To apply the DC voltage into the processing space, an electrode at ground potential (hereinafter referred to as the “grounding electrode”) having a surface thereof exposed to the processing space must be provided. However, in the case of carrying out the plasma processing using a reactive processing gas, reaction product (deposit) may become attached to the surface of the grounding electrode so that a deposit film is formed thereon. Such a deposit film is insulating, and hence the DC current flow from the upper electrode to the grounding electrode is impeded, so that the DC voltage can no longer be applied into the processing space; as a result, the plasma in the processing space may fall into an unstable state, or the plasma properties may change. It is thus necessary to remove such a deposit film from the grounding electrode.
Conventionally, as a method of removing a deposit film from an electrode surface, there has been known a method in which oxygen (O2) gas is introduced into the processing space, and oxygen ions and oxygen radicals are produced from the oxygen gas, so that the deposit film is removed through reaction with the oxygen ions and oxygen radicals (see, for example, Japanese Laid-open Patent Publication (Kokai) No. S62-040728).
However, for the above method of removing a deposit film, processing separate to the wafer plasma processing must be carried out, and hence there is a problem that the productivity of production of semiconductor devices from the wafers decreases. That is, there is a problem that removing the deposit film from the electrode surface is not practical from the viewpoint of productivity.